1. Field of the Invention
The invention relates to compatible polymer mixtures comprised of at least two different polymer components, one of which (P1) is a polystyrene, and the other (P2) is a copolymer of esters of methacrylic acid.
2. Discussion of the Background
As a rule, different polymer species are considered to be incompatible, i.e. they do not form a homogeneous phase except at very small concentrations of one of the components (a homogeneous phase being characterized by complete miscibility of the components).
Certain exceptions to this rule have attracted great interest, particularly among specialists concerned with the theoretical significance of the phenomena.
"Completely compatible" mixtures of polymers have complete solubility (miscibility) in all mixture ratios. The glass temperature, Tg, or the so-called "optical method" (clarity of a film cast from a homogeneous solution of the polymer mixture), is often used to demonstrate miscibility. (See Brandrup-Immergut, "Polymer Handbook", 2nd Ed., III:211-213; and 1982 "Kirk-Othmer Encyclopedia of Chemical Technology", 3rd Ed., pub. John Wiley & Sons, 18:443-478.)
An example of nearly complete incompatibility is the polymer system polystyrene/polymethyl methacrylate (PMMA). (See Olabisi, O., Robeson, L. M., and Shaw, M. T., 1979, "Polymer-Polymer Miscibility", pub. Academic Press, pp. 4 ff.) For example, for PMMA with molecular weight 160,000, miscibility is observed only up to 3.5 ppm PMMA in polystyrene (see Shaw, M. T., et al., 1984, Adv. Chem. Ser., 206:33-42). Polystyrene is also incompatible with polyethyl-, polypropyl-, polybutyl-, polyhexyl-, and polydecyl methacrylate, as well as with other polymethacrylic acid esters such as poly-3,3,5-trimethylcyclohexyl methacrylate and polyisobornyl methacrylate (see the unpublished Ger. Pat. App. No. P 36 32 369.1). The only exception is polycyclohexyl methacrylate, which is compatible with polystyrene in all mixture ratios over the entire accessible temperature range.
Mechanically produced mixtures of polymers (polyblends) have led to plastics products with improved properties, in particular cases and in specific areas of the plastics industry. (See Kirk-Othmer, loc. cit.) The physical properties of such polyblends ordinarily represent a compromise which generally cannot provide an improvement over the properties of the individual polymers. An example of a compatible polyblend of industrial importance is the system poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene, wherein the polyphenylene oxide component contributes a higher glass temperature Tg and improved impact strength, and the polystyrene component lowers the price of the product. Addition of the polystyrene also reduces the processing temperature of the polyphenylene oxide. (See Olabisi et al., loc. cit., pp. 14-15). However, multiphase polymer mixtures have attained a much greater commercial importance than compatible mixtures (see Kirk-Othmer, loc. cit., p. 449). There are sharp differences between the physical properties, particularly technically important properties such as optical properties (transparence, clarity, etc.), of multiphase (incompatible) polymer mixtures and those of compatible polymer mixtures. Compatible polymer mixtures notably differ from multiphase mixtures by, e.g., the presence of a single glass temperature Tg. It is known that the glass temperature can affect other properties such as the variation of shear modulus of elasticity with temperature, creep properties, viscosity, crystallization behavior of the plastic, etc. (see Olabisi et al., loc. cit., pp. 321-358). Also, as mentioned, incomplete compatibility frequently imposes narrow limits on any attempt to improve the overall property spectrum of a plastic.